An Aneurysm is the focal abnormal dilation of an artery. The complications which arise from aneurysms are specifically rupture, embolisation, fistularisation and symptoms related to pressure on surrounding structures. Aneurysms are commonly found in the abdominal aorta, being that part of the aorta which extends from the diaphragm to the point at which the aorta bifurcates into the common iliac arteries. These abdominal aortic aneurysms typically occur between the point at which the renal arteries branch from the aorta and the bifurcation of the aorta.
The standard treatment for aneurysms is to resect them by opening the aneurysm directly and inserting an inlaid graft made of a biocompatible material such as Dacron. The operation in most cases is large entailing considerable blood loss, at least 10 days in hospital and a mortality rate of about 5% in elective cases. This mortality rate is normally related to associated vascular problems such as myocardial infarction. Many patients cannot be subjected to such a large procedure because of intercurrent disease and therefore die of the aneurysm or the complications thereof.
It has been proposed by Balka etal. (Journal of Surgical Research 40 305-309 (1986)) to treat abdominal aortic aneurysms by the insertion of an intraluminal prosthesis, which approximates the diameter of the aorta above and below the aneurysm, into the aorta through the common femoral artery. In this case the prosthesis comprised a polyurethane tube with a nitinol and/or stainless steel frame which was designed in such a configuration that it can be compressed inside a catheter and then regain its original shape after being discharged into the aorta. This proposal does not appear to have been adopted for the treatment of humans due to difficulty in ensuring that the prosthesis will expand sufficiently to form a seal with the aorta above and below the aneurysm. The present inventor has developed a prosthesis which provides an alternative to that proposed by Balka et al.
In a first aspect the present invention consists in a self expanding vascular endoprosthesis adapted to bridge across an aneurysm in an artery, the endoprosthesis comprising a substantially imperforate sheet of a resiliently flexible biocompatible material, the sheet being rolled upon itself about one of its longitudinal edges, the material from which the sheet is formed being such that
(a) upon being introduced into an artery the endoprosthesis will resiliently expand of its own volition to press firmly against the internal wall of the artery to bridge across the aneurysm and to fluid isolate it from blood flowing in the artery, and PA1 (b) the endoprosthesis has sufficient longitudinal stiffness that there will be a compliance mismatch between the endoprosthesis and the wall of the artery to induce sufficient cellular proliferation in that wall adjacent the ends of an implanted endoprosthesis to cause the endoprosthesis to be adhered to the arterial wall.
In a second aspect the present invention consists in apparatus for introducing a self expanding vascular endoprosthesis for aneurysms into an artery, comprising an elongate tubular catheter, a self expanding vascular prosthesis for aneurysms according to the present invention disposed within the catheter and means for ejecting the endoprosthesis from the catheter.
In a third aspect the present invention consists in a method for treating an aneurysm in an artery by introducing a self expanding endoprosthesis into the artery, the method comprising the steps of:
inserting one end of a catheter containing a self expanding vascular endoprosthesis according to any one of claims 1 to 5 into an artery communicating with the artery having the aneurysm,
moving the catheter along the patient's vascular system until the end of the catheter is adjacent the aneurysm,
ejecting the endoprosthesis from one end of the catheter such that it bridges across the aneurysm and expands firmly into contact with the wall of the artery so that the aneurysm is fluid isolated from blood flowing in the artery, and
causing the endoprosthesis to be held in position bridging across the aneurysm by cellular proliferation of the wall of the artery caused by the compliance mismatch between the endoprosthesis and the wall of the artery.
The endoprosthesis is preferably formed from a substantially rectangular sheet of a suitable grade of polypropylene or another similar synthetic plastics material. The sheet preferably has a thickness of from 0.01 mm to 0.8 mm, more preferably 0.3 mm to 0.5 mm. The corners of the sheet which are on the outside of the prosthesis are preferably rounded to avoid ulceration of the arterial wall. The length of the sheet must be sufficient to bridge the aneurysm but is preferably sufficient that one end rests against a bifurcation of the artery in which the aneurysm occur. This latter preferment assists in retention of the endoprosthesis in a position in which it bridges over the aneurysm. still allowing blood supply to the diverging vessels.
In another embodiment of the invention the endoprosthesis is such that upon release from the end of the catheter it is capable of increasing in length as well as expanding radially outwardly. The sheet forming the endoprosthesis might have a "memory" causing it to want to expand from its rolled up cylindrical form into a helical form of greater diameter than the initial cylinder and of greater length. The overlapping turns of the expanded helical coil serving to prevent fluid communication between the interior of the endoprosthesis and the aneurismal sac. In another form of the invention the sheet forming endoprosthesis may be of a very thin film having ribs which assume a helical form when released from the endoprosthesis. The advantage of an endoprosthesis which can increase in length after release from the catheter is that it is easier to thread a catheter containing such a shortened endoprosthesis through the patient's vascular system to the point of the aneurysm.
The sheet of material from which the endoprosthesis is rolled up preferably has a compliance mismatch with the vascular tissue and is preferably quite stiff in a longitudinal direction. This is believed to have the effect of stimulating a reaction in the arterial wall and thereby inducing cellular proliferation in the vascular tissue surrounding the ends of the endoprosthesis. This causes a proliferation of endothelial cells which has the effect of adhering the endoprosthesis to the arterial wall. The endoprosthesis thus has a self suturing effect which retains it against movement along the artery.
The material from which the endoprosthesis is formed should be resiliently flexible so that upon being released from the constraint of the catheter the prosthesis will expand to bear against the arterial wall above and below the aneurysm. The use of the sheet of material rolled up along one of its side edges to form a scroll has been found to allow the prosthesis to expand very considerably if need be. This feature is important because the neck of the aneurysms tend to vary greatly between patients. Also depending upon where the ends of the endoprosthesis extend to the size of the native artery may be quite small or quite large. It is important that the endoprosthesis does not occlude vessels extending laterally from the artery and thus it may be necessary to terminate the endoprosthesis in a mildly distended part of the aneurysm. For this reason it may be necessary for the endoprosthesis to expand not merely to the normal diameter of the artery but to whatever extent is necessary to form a seal with the artery at either end of the aneurysm so that systalic blood pressure is not transmitted to the aneurysmal sac formed between the endoprosthesis and the distended arterial wall.
In the case of the abdominal aorta the normal internal diameter of the aorta is about 18 mm. Abdominal aortic aneurysms will typically have a diameter of from 40 to 70 mm. The abdominal aorta between the renal arteries and the iliac arterial bifurcation is typically about 110 mm. The aneurysm normally extends along a substantial portion of the abdominal aorta and is bounded at either end by a neck of undistended arterial wall adjacent the renal arteries and adjacent the iliac arterial bifurcation. In this case then the prosthesis is preferably rolled up from a sheet of polypropylene having a thickness of 0.4 mm, a length of 110 mm and a width of from 98 mm to 142 mm. It should be recognized however that the neck of the aneurysms tend to be very variable and it may be necessary to use a sheet wider than that indicated to form the endoprosthesis.
The present inventor has found that the endoprosthesis according to the present invention may be rolled up to a very small diameter allowing its introduction into a deep artery, such as the abdominal aorta, from a more superficial but much smaller artery, such as the common femoral artery.
The apparatus according to the present invention comprises a conventional catheter into which the endoprosthesis has been inserted in a rolled up condition and means to eject the endoprosthesis from an end of the catheter. The apparatus may also include a guide wire and/or sensing means to assist in the determination of the correct position at which the endoprosthesis should be ejected from the catheter. The ejection of the endoprosthesis from the catheter may be achieved by holding the catheter stationary and pushing the endoprosthesis from it using a plunger extending down the catheter or the plunger may be abutted against the proximal end of the endoprosthesis and the catheter withdrawn from around the endoprosthesis.